1. Technical Field
The present disclosure relates to a microelectromechanical gyroscope with compensation of quadrature error drift.
2. Description of the Related Art
As is known, use of microelectromechanical systems is becoming more and more widespread in several fields of technology and has lent encouraging results, especially in manufacturing of inertial sensors, microintegrated gyroscopes and electromechanical oscillators for various applications.
MEMS of this type are generally based on microelectromechanical structures comprising a supporting body and at least on movable mass, coupled to the supporting body through flexures. Flexures are configured to allow the movable mass to oscillate with respect to the supporting body in accordance with one or more degrees of freedom. The movable mass is capacitively coupled to a plurality of static electrodes on the supporting body, thus forming capacitors with variable capacitance. The movement of the movable mass with respect to the static electrodes on the supporting body, e.g., on account of external forces, modifies the capacitance of the capacitors; hence, it is possible to trace back to the displacement of the movable mass in respect of the supporting body and to the force applied. Vice versa, when appropriate bias voltages are provided, possibly through a separate set of driving electrodes, it is possible to apply an electrostatic force to the movable mass to set it in motion. Moreover, in order to provide microelectromechanical oscillators, it is conventional to exploit the frequency response of the MEMS structures, which is of the low-pass, second order type, with a resonance frequency.
The MEMS gyroscopes, in particular, have a more complex electromechanical structure, which, typically, comprises two masses movable with respect to the supporting body and coupled to each other in such a way as to leave one relative degree of freedom. The two movable masses are both capacitively coupled to the supporting body through static sensing and/or driving electrodes. One of the masses is dedicated to driving and is kept in oscillation at the resonance frequency with controlled oscillation amplitude. The other mass is drawn in the oscillatory motion (either translational or rotational) and, in case of rotation of the microstructure about a gyroscopic axis with an angular rate, is subjected to a Coriolis force that is proportional to the angular rate itself. In practice, the drawn mass acts as an accelerometer that allows to detect Coriolis force and to trace back to the angular rate. In some cases, a single mass is coupled to the supporting body so as to be movable with respect to the supporting body with two independent degrees of freedom. A driving device maintains the movable mass in controlled oscillation according to one of the two degrees of freedom. The movable mass moves in accordance with the other degree of freedom in response to a rotation of the supporting body about a sensing axis, on account of Coriolis force.
In order to properly operate, a MEMS gyroscope requires a driving device that maintains the movable mass in oscillation at the resonance frequency, and a reading device, to detect displacements of the drawn mass. These displacements are representative of the Coriolis force and of the angular rate are detectable through reading electric signals correlated to variations of the capacitance between the drawn mass and the static electrodes. Because of driving at the resonance frequency, the reading signals are in the form Dual Side Band-Suppressed Carrier (DSB-SC) signals. The carrier signal is defined by the oscillation velocity of the driving mass, at the mechanical resonance frequency.
However, the MEMS gyroscope has a complex structure and electromechanical interactions between the movable masses and supporting body are often non-linear, the useful signal components are mixed with spurious components, that do not contribute to measurement of the angular speed. Spurious components may depend on several causes. For example, manufacturing defects and process spreads are virtually unavoidable sources of disturbance, the effect of which is not predictable.
A common defect depends on the fact that the direction of oscillation of the driving mass is not perfectly coincident with the intended design degree of freedom. Such a defect is normally due to imperfections in the elastic connections between the movable mass and the supporting body. This defect causes quadrature errors, i.e., signal components of unknown amplitude at the same frequency as the carrier and an 90° out of phase.
Quadrature components are in most cases so large that they cannot be simply neglected without introducing significant errors. Normally, factory calibration at the end of the manufacturing process allows to reduce errors within acceptable margins. However, the problem is not completely solved, because the amplitude of the quadrature components may vary during device lifetime. In particular, the supporting body may be deformed because of mechanical stress or temperature variations. In turn, deformations of the supporting body cause unpredictable variations in the movements of the masses and, consequently, in the quadrature components, that are no longer effectively compensated.